1.1 Field of the Invention
The present invention discloses transgenic plants expressing substantially higher levels of insect controlling Bacillus thuringiensis δ-endotoxin. Methods for obtaining such plants and compositions, and methods for using such plants and compositions are described. Also disclosed are improved polynucleotide cassettes containing preferred protein coding sequences which impart the substantially higher levels of insect controlling δ-endotoxins. The preferred embodiments of the invention surprisingly provide up to ten fold higher levels of insect controlling protein-relative to the highest levels obtained using prior compositions. In particular, transgenic maize expressing higher levels of a protein designed to exhibit increased toxicity toward Coleopteran pests deliver superior levels of insect protection and are less likely to sponsor development of populations of target insects that are resistant to the insecticidally active protein.
1.2 Description of the Related Art
Almost all field crops, plants, and commercial farming areas are susceptible to attack by one or more insect pests. Particularly problematic are Coleopteran and Lepidopteran pests. Because crops of commercial interest are often the target of insect attack, environmentally-sensitive methods for controlling or eradicating insect infestation are desirable. This is particularly true for farmers, nurserymen, growers, and commercial and residential areas which seek to control insect populations using ecologically friendly compositions.
The most widely used environmentally-sensitive insecticidal formulations developed in recent years have been composed of microbial protein pesticides derived from the bacterium Bacillus thuringiensis, a Gram-positive bacterium that produces crystal proteins or inclusion bodies which are specifically toxic to certain orders and species of insects. Many different strains of B. thuringiensis have been identified which produce one or more insecticidal crystal proteins as well as other insecticidal non-crystal forming proteins. Compositions including B. thuringiensis strains which produce insecticidal proteins have been commercially available and used as environmentally acceptable insecticides because they are quite toxic to specific target insect pests, but are harmless to plants and to vertebrate and invertebrate animals. More importantly, because these insect controlling proteins have to be ingested by susceptible target insect pests in order to exert their insecticidal or toxic effects judicious application of such protein compositions limits or prevents non-target insect members of the susceptible order which may also be susceptible to the composition from significant exposure to the proteins (for example, non-target Lepidopteran species where Lepidopteran specific B.t. crystal protein is used in an insecticidal formulation). Additionally, insects of various orders have been shown to totally lack susceptibility to specifically targeted insecticidal proteins even when ingested in large amounts.
1.2.1 δ-Endotoxins
δ-endotoxins are used to control a wide range of plant-eating caterpillars and beetles, as well as mosquitoes. These proteins, also referred to as insecticidal crystal proteins, crystal proteins, and Bt toxins, represent a large collection of insecticidal proteins produced by B. thuringiensis that are toxic upon ingestion by a susceptible insect host. Over the past decade research on the structure and function of B. thuringiensis toxins has covered all of the major toxin categories, and while these toxins differ in specific structure and function, general similarities in the structure and function are assumed. A recent review describes the genetics, biochemistry, and molecular biology of Bt toxins (Schnepf et al., Bacillus thuringiensis and its Pesticidal Crystal Proteins, Microbiol. Mol. Biol. Rev. 62:775-806, 1998). Based on the accumulated knowledge of B. thuringiensis toxins, a generalized mode of action for B. thuringiensis toxins has been created and includes: ingestion by the insect, solubilization in the insect midgut (a combination stomach and small intestine), resistance to digestive enzymes sometimes with partial digestion by gut specific proteases catalyzing specifically a cleavage at a peptide site within a protoxin structure which “activates” the toxin, binding of the toxin to the midgut cells' brush border, formation of a pore in the insect midgut cell, and the disruption of cellular homeostasis (English and Slatin, 1992).
1.2.2 Genes Encoding Crystal Protiens
Many of the δ-endotoxins are related to various degrees by similarities in their amino acid sequences. Historically, the proteins and the genes which encode them were classified based largely upon their spectrum of insecticidal activity. A review by Höfte and Whiteley (1989) discusses the genes and proteins that were identified in B. thuringiensis prior to 1990, and sets forth the nomenclature and classification scheme which has traditionally been applied to B. thuriringiensis genes and proteins. The original nomenclature took advantage of the discovery that the few Bt Cry proteins known at the time generally fell into a limited number of classes, wherein each class represented proteins having specificity for specific orders of insects. For example, cry1 genes encoded Lepidopteran-toxic Cry1 proteins. cry2 genes encoded Cry2 proteins that were generally toxic to both Lepidopterans well as to Dipterans. cry3 genes encoded Coleopteran-toxic Cry3 proteins, while cry4 genes encoded Dipteran-specific toxic Cry4 proteins. The nomenclature has, for the past decade or more become rather confusing with the discovery of more distantly related classes of insecticidal Bt proteins. More recently, a simplified homogeneous nomenclature and basis for classifications of Bt proteins has been adopted and has been reviewed by Schnepf et al. (1998). Schnepf et al. (1998) also provides a structural solution for a Cry1 crystal. This simplified nomenclature will be adopted herein. The convention of identifying Bt genes with lower case, italicized letters (eg. cry1Ab1) and identifying Bt proteins with uppercase first character (eg. Cry1Ab1) will also be observed herein.
Based on the degree of sequence similarity, the proteins have been further classified into subfamilies. Proteins which appeared to be more closely related within each family were assigned divisional letters such as Cry1A, Cry1B, Cry1C, etc. Even more closely related proteins within each division were given names such as Cry1Ca, Cry1Cb, etc. and still even more closely related proteins within each division were designated with names such as Cry1Bb1, Cry1Bb2, etc.
The modern nomenclature systematically classifies the Cry proteins based upon amino acid sequence homology rather than upon insect target specificities. The classification scheme for many known toxins, not including allelic variations in individual proteins, is summarized in regularly updated tables which can be obtained from Dr. Neil Crickmore at the biology department of Sussex University in Great Britain.
1.2.3 Bio-insecticide Polypeptide Compositions
The utility of bacterial crystal proteins as insecticides was extended beyond Lepidopterans and Dipteran larvae when the first isolation of a Coleopteran-toxic B. thuringiensis strain was reported (Krieg et al., 1983; 1984). This strain (described in U.S. Pat. No. 4,766,203, specifically incorporated herein by reference), designated B. thuringiensis var. tenebrionis, was reported to be toxic to larvae of the Coleopteran insects Agelastica alni (blue alder leaf beetle) and Leptinotarsa decemlineata (Colorado potato beetle).
U.S. Pat. No. 5,024,837 also describes hybrid B. thuringiensis var. kurstaki strains which showed activity against Lepidopteran insects. U.S. Pat. No. 4,797,279 (corresponding to EP 0221024) discloses a hybrid B. thuringiensis containing a plasmid from B. thuringiensis var. kurstaki encoding a Lepidopteran-toxic crystal protein-encoding gene and a plasmid from B. thuringiensis tenebrionis encoding a Coleopteran-toxic crystal protein-encoding gene. The hybrid B. thuringiensis strain produces crystal proteins characteristic of those made by both B. thuringiensis kurstaki and B. thuringiensis tenebrionis. U.S. Pat. No. 4,910,016 (corresponding to EP 0303379) discloses a B. thuringiensis isolate identified as B. thuringiensis MT 104 which has insecticidal activity against Coleopterans and Lepidopterans. More recently, Osman et al. disclosed a natural Bacillus thuringiensis isolate which displayed activity against at least two orders of insects and against nematodes (WO 98/30700).
It has been known for more than two decades that compositions comprising Bt insecticidal proteins are effective in providing protection from insect infestation to plants treated with such compositions. More recently, molecular genetic techniques have enabled the expression of Bt insecticidal proteins from nucleotide sequences stably inserted into plant genomes (Perlak et al., Brown & Santino, etc.). However, expression of transgenes in plants has provided an avenue for increased insect resistance to Bt's produced in plants because plants have not been shown to produce high levels of insecticidal proteins. It was initially believed that gross morphological or topological differences in gene structure and architecture between plant and bacterial systems was the limitation which prevented over-expression of Bt transgenes in plants. These differences were seemingly overcome as disclosed by Perlak et al. (U.S. Pat. No. 5.500,365) and by Brown et al. (U.S. Pat. Nos. 5,424.412 and 5.689.052) wherein transgenes encoding Bt insecticidal protein which contained plant preferred codons were shown to improve the levels of expression. Alternatively, truncating the protoxin coding domain to the shortest peptide coding domain which still encoded an insecticidal protein was also deemed sufficient to overcome the limitation of vanishingly low expression levels of the Bt encoding transgene in planta. Expression levels of Bt proteins in planta from transgenes has varied widely independent of the means used for expression, and accumulated protein levels have ranged from virtually undetectable to 2 parts per million to around 20 to 30 parts per million. However, even though all of these approaches provided improved levels of Bt protein accumulation in plants, none provided levels of expression which could ensure that insect resistance would not become a problem without the necessity of coordinate expression of one or more additional insecticidal toxins by the transgenic plant, or alternatively without the coordinate topical application of additional supplemental Bt or insecticidal chemical compositions.
The importance of accumulation of higher levels of Bt toxin for preventing insect resistance to individual Bt toxins has been understood for some time. Various laboratory studies in which selection against Bt was applied over several generations of insects have confirmed that resistance against Bt insecticidal proteins is seldom obtained. It should be emphasized that laboratory conditions represent rather low but constant selection pressure conditions, allowing for the survival of a sub-population of insects which have been subjected to insecticidal pressure and which produce the subsequent generations of insects. Succeeding generations are also maintained on media containing low but constant concentrations of insecticidal protein. Generally, concentrations used for selection pressures range from LC40 to around LC60 or so, however, LC95 concentrations have also tested for the development of resistance. In most cases, resistance is acquired slowly, generally developing within a reasonably few generations, for example 10-50 generations. However, such resistance is not observed where substantially higher levels of toxin are used, or in situations in which multiple toxins are provided.
At present, recombinant plants expressing commercially useful levels of Bt insecticidal protein generally contain only one gene encoding a single class of Bt. Such plants are anticipated to have a very limited duration of use for two reasons. First, these plants are expressing insufficient levels of the insecticidal protein to ensure that all target insects exposed to and feeding from the plant tissues will succumb due to the dose of toxin ingested. Second, because of the insufficient insecticidal protein levels, the potential for development of resistance is unreasonably increased. This is not to say that the level of toxin produced by such transgenic plants is insufficient to be effective. This merely represents the limitations of expression of δ-endotoxins in planta even when using sequences encoding Bt δ-endotoxin which have been modified to conform to plant preferred sequences. One limitation which has been observed for many Bt δ-endotoxin encoding sequences modified for expression in plants is that is has been impossible to predict which Bt δ-endotoxin would be effective for expression in plants. (For example, expression of Cry2Aa in cotton plants results in phytotoxicity when targeted to the chloroplast, however expression of a closely related cry2Ab sequence is not phytotoxic when targeted to the chloroplast. (Corbin et al., U.S. patent application, Ser. No. 09/186,002). Even so, levels of δ-endotoxin protein produced in plants is not sufficient to be effective against all desired target insect species known to be susceptible to a given type and class of δ-endotoxin.
As indicated above, alternative approaches to development of resistance to insecticidal proteins has included ineffective attempts to increase the expression levels of transgenes in plants. Alternatively, additional insecticidal genes could be engineered into plants so that multiple toxins are coordinately expressed. This would provide a more effective means for delaying the onset of resistance to any one combination of Bt's, however, this still does not overcome the limitation of insufficient levels of insecticidal protein accumulating in the recombinant plant(s). An additional alternative to insufficient levels of expression has been to engineer genes encoding Bt insecticidal crystal proteins which demonstrate improved insecticidal properties, having either a broader host range or an increased biological activity, which could conceivably result in requiring less of the recombinant protein to control a target insect species than was required of the native form of the protein.
The combination of structural analyses of B. thuringiensis toxins followed by an investigation of the function of such structures, motifs, and the like has taught that specific regions of crystal protein endotoxins are, in a general way, responsible for particular functions.
Domain 1, for example, from Cry3Bb and Cry1Ac has been found to be responsible for ion channel activity, the initial step in formation of a pore (Walters et al., 1993; Von Tersch et al., 1994). Domains 2 and 3 have been found to be responsible for receptor binding and insecticidal specificity (Aronson et al., 1995; Caramori et al., 1991; Chen et al. 1993; de Maagd et al., 1996; Ge et al., 1991; Lee et al., 1992; Lee et al., 1995; Lu et al., 1994; Smedley and Ellar, 1996; Smith and Ellar, 1994; Rajamohan et al., 1995; Rajamohan et al., 1996; Wu and Dean, 1996). Regions in domain 2 and 3 can also impact the ion channel activity of some toxins (Chen et al., 1993, Wolfersberger et al., 1996; Von Tersch et al., 1994).
Unfortunately, while many investigators have attempted, few have succeeded in making mutated crystal proteins with improved insecticidal toxicity. In almost all of the examples of genetically-engineered B. thuringiensis toxins in the literature, the biological activity of the mutated crystal protein is no better than that of the wild-type protein, and in many cases, the activity is decreased or destroyed altogether (Almond and Dean, 1993; Aronson et al., 1995; Chen et al., 1993, Chen et al., 1995; Ge et al., 1991; Kwak et al., 1995; Lu et al., 1994; Rajamohan et al., 1995; Rajamohan et al., 1996; Smedley and Ellar, 1996; Smith and Ellar, 1994; Wolfersberger et al., 1996; Wu and Aronson, 1992). However, Van Rie et al. have recently accomplished the improvement of a Cry3A δ-endotoxin having increased Coleopteran insecticidal activity by identifying a single mutant having increased insecticidal activity. Van Rie et al. propose a method for identifying mutants having increased insecticidal activity in which the method consists of identifying amino acid mutations which decrease the insecticidal activity, and selectively altering those residues by site directed mutagenesis to incorporate one or more of the naturally occurring 20 amino acids at those positions, and feeding the various forms of the resulting altered protein to western or northern corn rootworms to identify those having improved activity (U.S. Pat. No. 5,659,123). While no sequences were enabled using the method, as mentioned above, Van Rie et al. succeeded in identifying only one sequence having increased activity and did not demonstrate an increase in expression of the mutant form as compared to the native sequence.
For a crystal protein having approximately 650 amino acids in the sequence of its active toxin, and the possibility of 20 different amino acids at each position in this sequence, the likelihood of arbitrarily creating a successful new structure is remote, even it a general function to a stretch of 250-300 amino acids can be assigned. Indeed, the above prior art with respect to crystal protein gene mutagenesis has been concerned primarily with studying the structure and function of the crystal proteins, using mutagenesis to perturb some step in the mode of action, rather than with engineering improved toxins.
Collectively, the limited successes in the art to develop non-naturally occurring toxins with improved insecticidal activity have stifled progress in this area and confounded the search for improved endotoxins or crystal proteins. Rather than following simple and predictable rules, the successful engineering of an improved crystal protein may involve different strategies, depending on the crystal protein being improved and the insect pests being targeted. Thus, the process is highly empirical.
Accordingly, traditional recombinant DNA technology is clearly not routine experimentation for providing improved insecticidal crystal proteins. What has been lacking in the prior art are rational methods for producing genetically-engineered B. thuringiensis crystal proteins that have improved insecticidal activity and, in particular, improved toxicity towards a wide range of Lepidopteran, Coleopteran, or Dipteran insect pests. Methods and compositions which address these concerns were disclosed in U.S. patent application Ser. No. 08/993,170 (Dec. 18, 1997; English et al.) and other related U.S. applications (Ser. No. 08/993,722, Dec. 18, 1997, English et al.; Ser. No. 08/993,755, Dec. 18, 1997, English et al.; and Ser. No. 08/996,441, Dec. 18, 1997, English et al.) and in Van Rie et al. (U.S. Pat. No. 5,659,123, Jun. 1, 1999). In addition, recombinantly improved δ-endotoxins have continued to be expressed poorly and/or cause phytoxic effects when expressed in plants, thus leading to the recovery of fewer commercially useful transgenic events.